The present invention pertains to a process for depositing a thin film of a II-VI compound cadmium zinc sulfide (CdZnS) by an aqueous solution growth technique. The film has a quality suitable for high efficiency heterojunction photovoltaic or other photoelectric devices which may benefit from the band edge shift
Heterojunction photovoltaic devices are cells wherein a junction is defined at an interface between two different semiconductor materials, one of which is n-type material such as CdS or CdZnS and the other of which is p-type material such as CuInSe.sub.2, or CuInGaSe.sub.2. One example of such prior art devices is disclosed in Mickelsen et al. U.S. Pat. No. 4,523,051 reissued as Re. 31,968.
One effective means of improving the efficiency rating of a heterojunction photovoltaic cell is to use layers of a II-VI, semiconductor such as CdS or the mixed CdZnS as the n-type layer in copper indium diselenide (CuInSe.sub.2)/CdZnS and copper indium gallium diselenide (CuInGaSe.sub.2)/CdZnS heterojunction devices. Since the n-type CdS or CdZnS is used as the window layer of the solar cell, the transparency of the CdS or CdZnS layers is critical over the full range from the band edge of the absorber on the long wavelength end (1.3 microns) to the cutoff of the solar spectrum on the short wavelength end (0.3 microns).
When used in heterojunction photovoltaic devices, the sulfide layers have been deposited in thicknesses of approximately 3 microns and show essentially complete absorption below a wavelength of approximately 500 nm, depending on the Zn content of the sulfide.
It has been shown that by increasing the Zn content in the CdZnS from 0% for CdS to 20% for Cd.sub.0.8 Zn.sub.0.2 S the band edge of this material is shifted to higher energies, extrinsic near-band-edge absorption is reduced and the open circuit voltage and resulting efficiency of CuInSe.sub.2 /CdZnS and CuInGaSe.sub.2 /CdZnS devices is increased.
It has also been shown that by decreasing the thickness of the sulfide layer below a critical value in the range of 50 nm, an increase in radiation transmission through the sulfide and therefore an increase in the energy conversion efficiency in the CuInSe.sub.2 /CdZnS and CuInGaSe.sub.2 /CdZnS devices is obtained. The decreased thickness of the sulfide layer reduces the absorption of incoming light in the sulfide layer through which the light must pass before reaching the active junction area. However, it has proved difficult to obtain a CdZnS layer that is sufficiently transparent to allow all of the radiation transmission through the sulfide. In the past, formation of CdZnS layers has been by use of physical vapor deposition from the compounds in a vacuum. Vapor deposition has resulted in layers which are too thick to provide maximum energy conversion efficiency in heterojunction devices.
The Choudary et al. U.S. Pat. No. 4,611,091, discloses a similar cell where deposition of CdS by a solution growth technique has been proposed but apparently never used. Chemical deposition of CdS films has been described in the article, "Some Properties of Thin Films of Chemically Deposited Cadmium Sulphide" by Danaher et al. in Solar Energy Materials Vol. 12 (1985), pp. 137-148, and of Cd.sub.(1-x) Z.sub.x S in the article, "Electroless Deposition of Semiconductor Films" by Sharma et al, Thin Solid Films, Vol. 60 (1979), pp. 55-49. However, solution growth of sulfide layers suitable for specialized photovoltaic devices has not been described and there is no indication that such solution growth procedures would be applicable to forming a CdZnS film that is sufficiently thin to pass a substantial portion of the radiation having a wavelength of 400 nm and yet be sufficiently continuous to avoid contact between the layers on opposite sides of the CdZnS film.